As I understand it, this is based on the Ubuntu LTS kernel, but doesn't necessarily rely on upstream's release schedule for fixes.

Given that most of the Proxmox userland comes from Debian Stable repos directly, it's probably not the worst idea in the world if you want to run ZFS on mostly-Debian Stable.

Oh, and while I'm not sure when the fix was applied, CVE-2026-98432 is patched on my Proxmox (no subscription, all ZFS including root) server.

Are you using the vanilla Debian kernel? The advantage of Ubuntu is that you don't need to depend on DKMS for something as vital as a file system.